barrel distorsion

if you are struggling to do this try latest version of DPP, there are lens correction features included (I have not used them, just know they are there)
 
In what context?
Barrel distortion () results from the compromise(s) made in lens design, most typically, I guess, in designing short focal length range zoom lenses - or wide angle prime lenses.
More expensive examples are often better corrected.
Considering a zoom lens, there is usually a focal length in the range which exhibits the phenomenon least. With a prime, of course, you don't have that option.
Typically exhibited at the edges of the frame, cropping can help.
It can also be corrected to a degree in PP.
 
Prevented? Better lens design. By the time it's at the end consumer, it basically boils down to spending more on a better designed lens (with a few exceptions).

There is software that can remove lens distortions - I have a feeling there's some sort of distortion reducer built into PhotoShop Elements since PSE7.
 
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